Hyperhomocysteinaemia and vascular injury: advances in mechanisms and drug targets

Abstract

Homocysteine is a sulphur-containing non-proteinogenic amino acid. Hyperhomocysteinaemia (HHcy), the pathogenic elevation of plasma homocysteine as a result of an imbalance of its metabolism, is an independent risk factor for various vascular diseases, such as atherosclerosis, hypertension, vascular calcification and aneurysm. Treatments aimed at lowering plasma homocysteine via dietary supplementation with folic acids and vitamin B are more effective in preventing vascular disease where the population has a normally low folate consumption than in areas with higher dietary folate. To date, the mechanisms of HHcy-induced vascular injury are not fully understood. HHcy increases oxidative stress and its downstream signalling pathways, resulting in vascular inflammation. HHcy also causes vascular injury via endoplasmic reticulum stress. Moreover, HHcy up-regulates pathogenic genes and down-regulates protective genes via DNA demethylation and methylation respectively. Homocysteinylation of proteins induced by homocysteine also contributes to vascular injury by modulating intracellular redox state and altering protein function. Furthermore, HHcy-induced vascular injury leads to neuronal damage and disease. Also, an HHcy-activated sympathetic system and HHcy-injured adipose tissue also cause vascular injury, thus demonstrating the interactions between the organs injured by HHcy. Here, we have summarized the recent developments in the mechanisms of HHcy-induced vascular injury, which are further considered as potential therapeutic targets in this condition.

Linked articles: This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Figure 1

(A) Schematic overview of homocysteine metabolism changes in HHcy induced by re‐methylation (i) or trans‐sulphuration (ii) deficiency. Dietary deficiency of folate (THF) and vitamin B12 or a MTHFR dysfunctional mutation inhibits the re‐methylation of homocysteine into Met and induces homocysteine accumulation. Low levels of Met decrease SAM production, while elevated homocysteine levels contribute to SAH production, catalysed by S‐adenosylhomocysteine hydrolase (SAHH). Thus, the MT‐mediated transformation of SAM into SAH markedly decreases, leading to the lack of methyl donors. (B) Insufficient intake of vitamin B6 or CBS deficiency impairs the trans‐sulphuration of homocysteine and leads to the elevation of this amino acid. Increased levels of homocysteine induce enhanced SAH production, catalysed by SAHH. Because homocysteine re‐methylation is only marginally affected, SAM and methyl donor production is not significantly influenced. B6/B12, vitamin B6/B12.

Figure 2

Schematic overview of HHcy‐induced oxidative stress. HHcy up‐regulates NOX expression by Smad2/3 activation, while HHcy induces mitochondrial dysfunction via activating calcium signalling. Both these effects of HHcy result in superoxide production. Superoxide is converted to H₂O₂ catalysed by SOD. Moreover, HHcy up‐regulates iNOS and enhances eNOS uncoupling, which further induces peroxynitrite formed by NO and superoxide. Superoxide, H₂O₂ and peroxynitrite all are ROS. HHcy also inhibits the activity of antioxidants, such as thioredoxin (Trx) and HO‐1, to up‐regulate NOX expression and attenuate antioxidant‐mediated elimination of ROS. NET, neutrophil extracellular trap.

Figure 3

NMDA receptors may act as the cell surface receptor for homocysteine. NMDA receptor mediates homocysteine‐induced ROS production by activating calcium signalling and impairing mitochondrial function. ROS accumulation further up‐regulates COX‐2 gene transcription. Homocysteine also leads to the nuclear translocation of β‐catenin through NMDA receptors. The nuclear translocation of β‐catenin results in disruption of vascular endothelial (VE)‐cadherin–β‐catenin interaction and inhibition of claudin‐5 transcription. Thus, the tight junction of cells is damaged as the homophilic interactions of VE‐cadherin and claudin‐5 are reduced.

Figure 4

Schematic overview of HHcy‐induced ER stress. HHcy‐induced ER stress is mediated by oxidative stress. Conversely, HHcy can directly contribute to ER stress, which further leads to oxidative stress. Cleaved ATF6, ATF4 and X‐box binding protein 1, spliced form (XBP1s) transcriptionally regulates their downstream genes separately. S1P, site 1 protease; S2P, site 2 protease; XBP1u, X‐box binding protein 1, unspliced form.

Figure 5

Schematic overview of HHcy‐induced epigenetic methylation. HHcy decreases SAM/SAH ratio and inhibits expression of DNMTs, thus leading to DNA hypomethylation and up‐regulation of pathogenic gene expression. In contrast, HHcy enhances DNMT expression and DNA methylation to down‐regulate expression of protective genes. Furthermore, HHcy up‐regulates tRNA MT NSun2 and promotes ICAM‐1 mRNA methylation, which further increases its gene expression. Histone H3‐Lys⁹ (H3K9) methylation protects against unstable atherosclerosis plaque. HHcy inhibits lysine MTs and H3K9 methylation, further inducing unstable plaque.

Figure 6

Chemical reactions of S‐homocysteinylation (A) and N‐homocysteinylation (B).

Figure 7

Mechanisms and drug targets of HHcy‐induced vascular injury. HHcy induces vascular injury by promoting oxidative stress, ER stress and protein homocysteinylation as well as regulating methylation. Among them, oxidative stress and ER stress aggravate mutually via ROS as a mediator. For homocysteinylation, the S‐linked reaction induces oxidative stress, whereas the N‐linked reaction regulates the property and activity of target proteins. Lowering homocysteine levels and inhibition of oxidative/ER stress are the potential therapeutic targets for drugs or natural chemicals. ALA, α‐lipoic acid; ECM, extracellular matrix; HQD, Huang Qi decoction; MT, metallothionein; SAL, salidroside.